It is known that a digital filter of order r permits, knowing the first discrete values r of an output digital function y and the most recent s+ 1 values of an input digital function x, of determining the new value of the output function: subword. ##EQU1##
If the filter is of the first order, the finite difference equation (1) can be written (i= 0; j= 1): EQU y(nT)= x(nT)+ K y [(n-1)T]
A digital filter therefore contains one or more multipliers of a binary function y or x by binary coefficients K.sub.i and L.sub.i.
A digital filter is the more interesting the larger is the number of channels it can process, i.e., the higher is the rate of the input signal to the filter. In this case, the samples or P.C.M. words of the input digital signal follow one another at a very rapid rate and it is necessary for a multiplication relating to a P.C.M. word to be terminated when the following word reaches the digital filter.
Referring by anticipation to FIG. 1, if one considers the digital filter comprising the adder 1, the multiplier 2 and the delay circuit 3 and if T is the duration of a frame, t.sub.+ the addition time in the adder 1, t.sub.* the multiplication time in the multiplier 2 and t the delay time of the delay circuit 3, one must have: EQU T.gtoreq. t.sub.+ + t.sub.* + t
If n is the number of channel time slots, the duration of a time slot is T/n and one must have EQU t.sub.+ .ltoreq. T/n EQU t.sub.* .ltoreq. T/n
With a conventional parallel-series multiplication the multiplication time of which is t.sub.* = 1 .mu.s and assuming T= 125 .mu.s, one can treat the maximum of EQU n.ltoreq. 125/1= 125 channels
In order to increase the number of channels, t.sub.+ and t.sub.* must be reduced.
Now, it can be observed that, if a word is applied to the input of the adder 1 at the moment 0, it leaves the adder 1 after addition at the moment t.sub.+ and the result of its multiplication by the coefficient K must appear on the other input of the adder 1 at the moment T. The computation time of the multiplication is therefore at the utmost t-t.sub.+.
Nevertheless, the words must apply to the multiplier 2 at the rate T/n. It will therefore be seen that in the digital filtering field, the result of the multiplication is not needed immediately and that what is important is not the computation time but the number of items of information treated per second.